The present invention relates generally to the creation of reticles. More specifically, the invention relates to creation of phase shift reticles.
A xe2x80x9cwaferxe2x80x9d is a thin piece of semiconductor material from which semiconductor chips are made. In various steps of forming semiconductor chips, photoresist masks may be formed. Errors in forming these masks may cause distortions that cause changes in the functions of these electronic devices. The photoresist masks may be used to create parts of the semiconductor device on the wafer surface in the dimensions required by the circuit design and to locate the parts in their proper location on the wafer surface. Design rule limitations frequently are referred to as critical dimensions. A critical dimension of a circuit is commonly defined as the smallest width of a line or the smallest space between two lines. The critical dimension is a limit to the smallest feature that can be reliably patterned by a given patterning process.
In a process of patterning a photoresist mask a photosensitive polymer film called photoresist is normally applied to a silicon substrate wafer and then allowed to dry. A lithographic tool may provide a light source passing through a reticle may be utilized to produce a light pattern on the photoresist, creating an exposed pattern of the photoresist. After exposure, the photoresist layer is developed to form a photoresist mask. The photoresist mask may then be used as a mask for etching, doping, depositing, or other processes in forming the semiconductor chips from the wafer.
An important limiting characteristic of the lithographic device is its resolution value. The resolution for a lithographic device is defined as the minimum feature that the lithographic device may consistently expose onto the wafer. When the resolution value of lithographic equipment is close to the critical dimension for an IC circuit design, the resolution of the lithographic device may influence the final size and density of the IC circuit. As the critical dimensions of the layout become smaller and approach the resolution value of the lithographic equipment, the consistency between the masked and actual layout pattern developed in the photoresist is significantly reduced. Specifically, it is observed that differences in pattern development of circuit features depend upon the proximity of the features to one another. The magnitude of such proximity effects depends on the proximity or closeness of the two features present on the masking pattern. Proximity effects are known to result from optical diffraction caused by the lithographic device. This diffraction causes adjacent features to interact with one another in such a way as to produce pattern-dependent variations. As a consequence of proximity effects, printed features do not have simple relationships to reticle dimensions. This creates a situation in which it is difficult to fabricate a photomask where the designer gets what he or she wants on the wafer.
Phase shifting often can significantly improve resolution by using destructive (rather than constructive) interference of the tail portions of the diffraction distributions from adjacent features on a mask. Various techniques have been proposed for phase shift, or optical proximity correction (OPC) mask fabrication.
These conventional techniques ordinarily are implemented in one of two ways. In one, a layer of phase shifting material is added to a reticle and then etched to produce the desired phase shift as in U.S. Pat. No. 5,741,613. In the other, the reticle itself is etched to a sufficient depth to produce the desired phase shift. The thickness and refractive index of the phase shifting material is sufficient to exactly shift the incident light by 180 degrees with respect to light that does not pass through the phase shifting material. One representative example is U.S. Pat. No. 6,197,456, where a reticle blank has a transparent layer and a non-transparent layer. Usually, the reticle blank has a transparent synthesized quartz substrate where multiple and different types of phase-shifting elements are etched into its surface.
Known methods may use a refractory metal, such as chrome, as a mask to etch the OPC pattern onto the phase-shift layer. Such refractory metals generally have a melting point that is greater than 1500xc2x0 C. Because of the very high melting point of refractory metals, relatively high-energy deposition methods, such as sputtering, may be required to deposit a film onto the reticle substrate. Beyond the additional logistical process difficulties and cost, an unfortunately consequence of high-energy deposition methods is the increased likelihood of damaging the transparent reticle substrate as it is bombarded by metal atoms with very high kinetic energy. It is generally more difficult to control the deposition of refractory metals on the substrate, usually resulting in films that are relatively thick, and not uniform. Moreover, relatively aggressive methods may be required to etch and remove refractory metals, which may result in further damage to substrate. Thus, inexpensive mask creation techniques using simple, easy to control agents and methods would be highly desirable.
To achieve the foregoing and other objects and in accordance with the purpose of the present invention, a method of forming a reticle is provided. In general, a metal containing material is vaporized through simple vaporization. The metal containing material is condensed on a substrate to form a metal containing layer on the substrate. A patterned photoresist layer is formed over the metal containing layer, defining exposed metal containing layer regions and covered metal containing layer regions. The metal containing layer in the exposed metal containing layer regions is removed from the substrate, while the metal containing layer in the covered metal containing layer regions remains on the substrate to form a metal containing mask. The substrate is plasma etched. The remaining metal containing layer is removed from the substrate.
In another embodiment of the invention a reticle is provided. In general, a metal containing material is vaporized through simple vaporization. The metal containing material is condensed on a substrate to form a metal containing layer on the substrate. A patterned photoresist layer is formed over the metal containing layer, defining exposed metal containing layer regions and covered metal containing layer regions. The metal containing layer in the exposed metal containing layer regions is removed from the substrate, while the metal containing layer in the covered metal containing layer regions remains on the substrate to form a metal containing mask. The substrate is plasma etched. The remaining metal containing layer is removed from the substrate.